The present invention relates to oriented polyester films having an improved metal adhesion promoting coating applied thereto. The improved coating composition permits rapid, non-sticking winding and unwinding while providing the same level of adhesion to subsequently-applied metallic layers as previous primed polyester films.
Metallic coated polyester films have utility in various packaging and solar control applications. The commercial acceptance of such films depends, inter alia, upon a good metallic layer/polyester film bond. Consequently, a body of art has developed which is directed to improving the adhesion of polyester films to subsequently applied metallic layers.
F. Funderburk et al, "Polyester Film Coated With Metal Adhesion Promoting Copolyester," U.S. Pat. No. 4,493,872 (Jan. 15, 1985) discloses a polyester film coated with a sulfonated copolyester coating composition. Polyester films covered by this patent have achieved extraordinary commercial success, as evidenced by outstanding sales. The Association of Industrial Metallizers, Coaters and Laminators (AIMCAL) awarded these films first place in its 1987 Technology of the Year Award.
Unfortunately, the adhesive nature of the sulfonated copolyester coating composition can create processing difficulties in the manufacture and subsequent metallization of these films. In particular, adjacent layers of these sulfonated copolyester coated films tend to stick together when wound into a roll. This sticking problem is most apparent when both sides of the film have been coated with the adhesive coating.
The severity of the sticking problem can be somewhat reduced by the inclusion of silica particles in the sulfonated copolyester coating composition. However, the addition of silica particles can create other problems, including the formation of undesirable coating patterns on the film surface. The silica can also alter the pH of the coating composition, thereby requiring the addition of a buffering agent, which may result in a shortened pot-life.
T. Oikawa et al, "Easily Adhesive Polyester Film," U.S. Pat. No. 4,699,845 (Oct. 13, 1987) broadly describes a polyester film having a coating layer on at least one surface which comprises a mixture of a higher fatty acid wax and a sulfonated copolyester. The addition of the higher fatty acid ester wax is said to improve the anti-blocking properties of the film. The coating can be applied as an aqueous dispersion between stretching operations. The "higher fatty acid wax" is defined as an aliphatic ester compound of a higher fatty acid having from 12 to 35 carbon atoms and an alcohol having from 15 to 35 carbon atoms. Carnauba wax, montan wax, wool wax and shellac wax are specifically disclosed as examples of the higher fatty acid wax. Carnauba wax and montan wax are especially preferred. The '845 patent also suggests that these higher fatty acid waxes can be modified by converting a part or all of the fatty acid ester to the corresponding alkali metal salt (such as sodium or potassium), or the corresponding alkali metal compound (such as calcium, magnesium or barium). The sulfonated copolyester is broadly disclosed as the reaction product of an aromatic dicarboxylic acid monomer and a glycol component in which part of the aromatic dicarboxylic acid is an alkali metal salt of an ester forming aromatic sulfonic acid such as 5-sodium sulfoisophthalate, sodium sulfoterephthalate and their ester forming derivatives. The dicarboxylic acid comonomer should comprise at least 60 mole percent of the sulfonated copolyester. The alkali metal salt of the ester-forming aromatic sulfonic acid should be present in an amount ranging from 7 to 40 mole percent, preferably 10 to 20 mole percent, of the total acidic reactants present.